1. Field of the Invention
This invention relates to direct positive silver halide photographic emulsions and, in particular, to an improvement in such emulsions spectrally sensitized with dimethine dyes.
2. Description of the Prior Art
When a silver halide photosensitive material is developed after exposure to light to which the material is spectrally sensitive, the emulsion layer blackens whereby the optical density increases with the increase in the amount of exposure, reaching a maximum. When the exposure amount surpasses the value corresponding to this maximum density, the density then decreases, thus finally giving a positive image from a negative. Such a phenomenon is generally referred to as solarization.
A similar phenomenon of density reversal is also observed with silver halides which have been either optically or chemically fogged during the manufacturing process of the emulsions. The term, "direct positive silver halide photographic emulsion" as used in the present specification means an emulsion which gives an optically positive image upon an ordinary image exposure to light followed by an usual development.
Many dyes are known which can be advantageously used for the spectral sensitization of usual negative silver halide emulsions. However, all of these dyes are not suited for the sensitization of direct positive emulsions. Use of these dyes in direct positive silver halide emulsions is sometimes accompanied by various disadvantages including a softening of the characteristic curve, or re-reversal phenomenon which refers to a density increase after the decrease of density in the solarization region. Moreover, the addition of such dyes to direct positive silver halide emulsions reduces the maximum density exhibited by the original emulsions.
Many dyes useful for spectral sensitization of direct positive silver halide emulsions are already known in the art. Patent specifications such as U.S. Pat. Nos. 3,314,796; 3,586,672; 3,314,796 and 3,598,596, and U.S. patent application Ser. No. 318,047, filed Dec. 26, 1972 have disclosed special usefulness of dimethine dyes containing indole nuclei.
It is also known from U.S. Pat. Nos. 3,592,653 and 3,598,603 that dimethine dyes containing pyrole nuclei are particularly useful for spectral sensitization of direct positive silver halide emulsions.
Further, the present inventors have already discovered that dimethine dyes having pyrazolo (1,5 - a) benzimidazole nuclei are particularly useful for the sensitization of direct positive silver halide emulsions as disclosed U.S. patent application Ser. No. 351,386, filed Apr. 16, 1973.
The inventors have also disclosed the usefulness of dimethine dyes having pyrazolo (5,1 - b) quinazolone nuclei for the sensitization of direct positive silver halide emulsions in U.S. patent application Ser. No. 379,887, filed Sept. 16, 1973.
On the other hand, U.S. Pat. No. 3,615,610 describes that the combined use of the above-described dimethine dyes and nitrostyryl dyes in direct positive silver halide emulsions results in an increased photographic speed, a reduced minimum density and an improvement in stability during storage.
Further, U.S. Pat. No 3,583,870 discloses that the combined use of sensitizing dyes excellent for the sensitization of negative silver halide emulsions and dipyridinium salt compounds in direct positive emulsions brings about an increase of spectral sensitivity as well as a reduction of the minimum density.
However, the most serious problem in the technology of direct positive silver halide photographic emulsions is the insufficient sensitivity for various photographic applications. Therefore, a sensitivity increase of direct positive silver halide emulsions, particularly by dye sensitization, is quite urgently desired.
An object of the present invention is to provide contrasty direct positive silver halide photographic emulsions which retain the maximum densities of the original emulsions and which exhibit high sensitivities as well as very low minimum densities.